The present invention relates to extreme ultraviolet lithography (EUVL), particularly to compensation of flare-induced CD changes in EUVL. More particularly, it relates to a method for substantially eliminating flare-induced CD changes by calculating flare and its variation over the area of a patterned mask and then using mask biasing to eliminate the CD variations caused by flare and its variations,
The scattering of light by the components of lithographic cameras is a problem of ever-increasing importance for the semiconductor industry. Scattering causes the redirection of light from an area of an image intended to be bright into all areas of the image, including those regions intended to be dark. The resulting background illumination is called xe2x80x9cflare,xe2x80x9d which reduces image contrast and the process window for printing. More importantly, flare also has a detrimental effect on the dimensions of critical features, referred to as critical dimensions (CD), and thus has a detrimental effect on CD control; localized flare variations (which are inevitable), lead to localized CD variations. In the manufacture of semiconductor devices, such as microprocessors, it is extremely important that the CD""s are very accurately controlled.
The dimensions of critical circuit features or CD""s are defined by optical lithography and the associated processing steps. There are numerous effects that can cause CD""s to deviate from the desired targets. Among these is the background illumination or flare in the image projected by the camera, caused by light scattering within the camera. In optical lithography and deep ultraviolet (DUV) lithography (248-157 nm) the intrinsic flare of the camera varies over the image field; additionally, there are flare variations in the projected image that are caused by variations in the density of absorbers on the photomask. These flare variations cause CD variations. Because the intrinsic flare of optical and DUV cameras varies over the image field, it has not been possible in the past to compensate for flare-induced CD changes. Instead, emphasis has been placed upon reducing flare as much as possible.
In principle the compensation technique of this invention could be applied to optical and DUV, but its implementation may be so cumbersome as to be impractical. In optical and DUV one would need to compensate for across-field flare variations as well as within-die flare variations. In EUVL, flare is essentially constant across the field.
The intrinsic flare levels in extreme ultraviolet lithography (EUVL) cameras will most likely be higher than experienced in optical and DUV cameras. However, the intrinsic flare of EUVL cameras is essentially constant over the image field. Based on the recognition of this difference in intrinsic flare between optical and DUV cameras and EUV cameras, the method of the present invention makes it practical to compensate for the CD changes caused by flare and its variations caused by the absorber density variations on the photomask.
The compensation provided by the method of the present invention is accomplished by biasing the photomask, which means changing the dimensions of features on the photomask so that all features on the mask print within the desired CD range. Mask biasing is used today to compensate for CD variations caused by optical proximity effects, but has not been previously used to also correct for flare-induced CD variation. It is the recognition that the intrinsic flare of EUVL cameras is essentially constant over the image field that makes the use of the compensation techniques of the present invention practical. Basically the method of the invention involves calculating the flare and its variation over the area of a patterned mask that will be imaged and then using mask biasing to largely eliminate the CD variations that the flare and its variations would otherwise cause.
It is an object of the present invention to eliminate unwanted changes of critical dimensions or CD""s in extreme ultraviolet (EUV) lithography.
A further object of the invention is to provide compensation of flare-induced CD changes in EUVL.
Another object of the invention is to provide a method to eliminate flare-induced CD changes, particularly in EUVL systems.
Another object of the invention is to provide a method for reducing or eliminating undesirable CD changes which are flare-induced based on the realization that the intrinsic level of flare for an EUV camera is essentially constant over the image field.
Another object of the invention is to provide a method for compensation of flare-induced CD changes in EUV lithography which involves calculating the flare variation over the area of a patterned mask and then using mask biasing to largely eliminate the CD variations that the flare and its variations would otherwise cause.
Other objects and advantages of the present invention will become apparent from the following description and accompanying drawing. Basically the invention is directed to compensation of flare-induced CD changes in photolithography, particularly EUV lithography. Unlike optical lithography and DUV lithography, wherein the intrinsic flare varies over the image field, the intrinsic level of flare for an extreme ultraviolet (EUV) camera (the flare level for sub resolution opaque dot in a bright field mask) is essentially constant over the image field, (changing only very near the edges of the field). Based on this recognition, compensation of flare-induced CD changes in EUV cameras can be made in accordance with the method of the present invention, which basically involves calculating the flare and its variation over the area of a patterned mask that will be imaged and then using mask biasing to largely eliminate the CD variations that the flare and its variations would otherwise cause. The general method of this invention is, in principle, applicable to optical or DUV lithography but it is impractical to implement since the intrinsic flare for those lithographies is not constant over the image field.